Autofocussing systems for optical systems, such as microscopes, are known in the art. There are a number of different types, all of which include an autofocussing sensing unit and an autofocussing driver. The sensing unit provides a focus error signal which is proportional to the extent of defocus of the image (i.e. proportional to the distance between the nominal object plane and the actual plane of the object). The driver, which is typically a motor of some kind, translates either the object or the focal plane.
Autofocussing systems can be divided into two main groups, static and dynamic systems, based on the state of the object currently out of focus.
Static autofocus systems utilize the object, which remains stationary, to determine the extent of defocus. For example, the autofocus systems of most consumer cameras analyze the sharpness of the details of the object in a received image to determine the extent of defocus. As the optical system is moved toward and away from the object, the change in the defocus is measured.
A typical prior art microscope is illustrated in FIG. 1 to which reference is now made. The microscope includes, along an optical path 8, a translatable objective lens 10, a stationary tube lens 12, an image plane 14 which is the focal plane of the tube lens 12 and an object surface or plane 16 on which an object to be viewed, such as a film, is placed. The microscope additionally includes a light source 18 for illuminating the object and a beam splitter 20 for directing the light beam 22, produced by the light source 18, towards the object surface 16. The object on the object surface 16 reflects the light back through the objective lens 10, beam splitter 20 and tube lens 12 towards the image plane 14.
If the object surface 16 is not at the focal plane of lens 10, the translatable objective lens 10, the focus is adjusted by moving the lens 10 in the direction of the optical path 8. Alternatively, the object surface 16 is moved. The depth of field (DOF) of the objective lens 10 is limited, as illustrated in FIG. 1.
Microscopes typically have triangulating autofocussing sensing units which utilize oblique illumination and specular reflection. Thus, as shown in FIG. 2 to which reference is now made, an autofocussing light source 19 is placed so as to obliquely illuminate the object plane 16 and the microscope additionally includes a position sensing detector (PSD) 28 for sensing the lateral displacement of the beam (as will be described hereinbelow) and a motor 27 for moving objective lens 10.
In the microscope of FIG. 2, the light beam, labeled 30, is deflected by the beam splitter 20 at a point A to one side of the optical path 8. The deflected beam 30 is bent by the objective lens 10 so as to obliquely illuminate, at an angle a, the object plane 16 at a reflection point C.
Beam 30 is reflected as light beam 32 and is deflected by beam splitter 20 at a point B, on the other side of the optical path 8 from the point A. The deflected light beam 32 then illuminates the PSD 28 which measures the location at which beam 32 fell upon it.
If the object surface 16 is out of focus because it is far from the objective lens 10, for example, at the location labeled 16' and indicated by dashed lines, the light beam 30 will travel further before being reflected, to a reflection point D which is laterally shifted from the point C, the previous reflection point.
Since the distance between the reflection points C and D are a function of the extent of the defocus and since that distance is reflected in the distance between points B and B', the extent of the defocus can be measured and, accordingly, compensated by having motor 27 move objective lens 10.
The following U.S. patents provide descriptions of various systems operating on the above-described triangulation principle: U.S. Pat. No. 5,136,149 to Fujiwara et al. and U.S. Pat. No. 4,577,095 to Watanabe.
Dynamic autofocussing is utilized when it is desired to keep the object permanently in focus while moving relative to the objective lens. These methods are common in automatic optical inspection systems, such as for silicon wafer or reticle inspection systems, which must inspect a large object surface within a short period of time.
In automatic inspection systems, the object is continually being scanned (movement in the plane of the object surface noted by arrow 29) and the object typically has a two-dimensional pattern on it. As a result, the stationary autofocussing methods do not work since the movement of the object, and its pattern, affects the sensing of the focus. U.S. Pat. No. 4,639,587 to Chadwick et al. describes a system which bypasses the effects of a moving object. In their system, a grid which is in an illumination path is projected, through the objective lens of the microscope, onto an inspected article. The grid is alternately projected onto the article with two different offsets from the main optical axis of the microscope. As a result, the grid is projected obliquely onto the inspected article.
Each reflected beam is projected onto a static grid which is adjusted to the nominal grid image when the image is in focus by one quarter of the grid period. The light intensity of the reflected grids through the static grids is measured.
The displacement of the article to be inspected is determined as a function of the extent of displacement of the reflected grids with respect to the static ones. Since both optical paths provide the same grid displacement, but in opposite directions, the difference between their signals measures the focus error and, the focus error is insensitive to the pattern of the inspected article.
Since the error signal of U.S. Pat. No. 4,639,587 is produced by analog detectors, their system is very sensitive to background level of stray light, noise, and a lack of uniformity of the two optical paths. As a result, their system includes additional optical and electronic elements to provide the necessary self-compensation and self-calibration.
It is further noted that the triangulation methods require a fairly large angle of oblique illumination. Otherwise, the lateral shift of the beam will not be a sensitive enough measure of the extent of defocus. Therefore, triangulation methods do not work well for objective lenses with low numerical apertures which induce small angles of incidence of sensing beams.
U.S. Pat. No. 4,725,722 to Maeda et al. describes a method of autofocusing suitable for integrated circuits. The method projects a striped pattern onto the object to be focussed and the contrast of the image of the strip pattern is used for focusing. The image is imaged by an optical system and detected by two detectors, each in two different locations vis-a-vis the focal plane of the optical system. When the signals of the two detectors are equally out of focus, the object is at the focal plane of the optical system.